U.S. patent application number 10/090977 was filed with the patent office on 2002-10-10 for scanning receiver for use in power amplifier linearization.
Invention is credited to Bachman, Thomas A. II, Everline, Paul, January, David M., Kooker, Robert W., Sarver, Leonard, Shenenberger, Philip, Snyder, Robert, White, Paul E..
Application Number | 20020146996 10/090977 |
Document ID | / |
Family ID | 26782836 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020146996 |
Kind Code |
A1 |
Bachman, Thomas A. II ; et
al. |
October 10, 2002 |
Scanning receiver for use in power amplifier linearization
Abstract
A feed forward power amplifier system and method identify active
channels across a frequency band to suppress unwanted
intermodulation distortion (IMD) products in a communications
signal such as a single- or multi-carrier communications signal. A
scanning receiver identifies at least one active channel in a
frequency band, and identifies at least one portion of the
frequency band likely to include IMD products based upon the
identified active channel(s). Based upon the identified portion of
the frequency band, IMD products are suppressed from the
communications signal, e.g., by controlling the magnitude and/or
phase of a suppression signal mixed with the communications
signal.
Inventors: |
Bachman, Thomas A. II;
(Darlington, MD) ; Kooker, Robert W.; (Freeland,
MD) ; White, Paul E.; (York, PA) ; Everline,
Paul; (Wildwood, IL) ; Snyder, Robert; (New
Freedom, PA) ; January, David M.; (Parkton, MD)
; Sarver, Leonard; (York, PA) ; Shenenberger,
Philip; (Manheim, PA) |
Correspondence
Address: |
WOOD, HERRON & EVANS, L.L.P.
2700 Carew Tower
441 Vine St.
Cincinnati
OH
45202
US
|
Family ID: |
26782836 |
Appl. No.: |
10/090977 |
Filed: |
March 5, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60273659 |
Mar 6, 2001 |
|
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Current U.S.
Class: |
455/302 ;
455/296 |
Current CPC
Class: |
H03F 1/3229
20130101 |
Class at
Publication: |
455/302 ;
455/296 |
International
Class: |
H04B 001/10 |
Claims
What is claimed is:
1. An apparatus, comprising: (a) a signal path configured to
communicate an RF communications signal disposed in a frequency
band; and (b) a circuit arrangement configured to suppress
intermodulation distortion (IMD) products from the RF
communications signal by analyzing the signal path to identify at
least one active channel among a plurality of channels in the
frequency band, identifying at least one portion of the frequency
band likely to include IMD products based upon the identified
active channel(s), and suppressing from the RF communications
signal the IMD products at the identified portion of the frequency
band.
2. The apparatus of claim 1, wherein the plurality of channels in
the frequency band are each associated with a non-varying carrier
frequency.
3. The apparatus of claim 1, wherein the frequency band is the
Universal Mobile Telecommunications System (UMTS) frequency
band.
4. The apparatus of claim 1, wherein the circuit arrangement is
configured to identify each active channel by detecting an active
signal at a carrier frequency associated with such active
channel.
5. The apparatus of claim 4, wherein the circuit arrangement
includes a scanning receiver coupled to the signal path, the
scanning receiver configured to be tuned to a selected carrier
frequency and to output a power signal representative of the power
in the signal path at the selected carrier frequency.
6. The apparatus of claim 5, wherein the circuit arrangement is
configured to sequentially tune the scanning receiver to each of a
plurality of carrier frequencies, compare the power signal output
by the scanning receiver at each carrier frequency to a threshold,
and identify an active channel among the plurality of channels
based upon the comparison of the power signal output by the
scanning receiver when tuned to a carrier frequency associated with
such active channel to the threshold.
7. The apparatus of claim 5, wherein the circuit arrangement is
configured to determine whether the identified active channel(s)
constitute a valid channel configuration.
8. The apparatus of claim 5, wherein the circuit arrangement is
configured to store a value associated with the power signal at
each carrier frequency.
9. The apparatus of claim 5, wherein the circuit arrangement is
further configured to suppress the IMD products by tuning the
scanning receiver to the identified portion of the frequency band,
monitoring the power signal when the scanning receiver is tuned to
the identified portion of the frequency band, and adjusting at
least one of a phase and magnitude of a suppression signal applied
to the signal path so as to reduce the signal power in the signal
path within the identified portion of the frequency band.
10. The apparatus of claim 5, wherein the circuit arrangement is
further configured to identify a plurality of portions of the
frequency band likely to include IMD products, and to suppress the
IMD products by tuning the scanning receiver to each identified
portion of the frequency band, monitoring the power signal when the
scanning receiver is tuned to such identified portion of the
frequency band, and adjusting at least one of a phase and magnitude
of a suppression signal applied to the signal path so as to reduce
the signal power in the signal path within the plurality of
identified portions of the frequency band.
11. The apparatus of claim 1, wherein the RF communications signal
comprises a multi-carrier communications signal.
12. The apparatus of claim 1, wherein the at least one active
channel includes first and second channels respectively associated
with respectively associated with first and second carrier
frequencies, and wherein the circuit arrangement is configured to
determine the identified portion of the frequency band by
performing a calculation selected from the group consisting of
subtracting a spectral distance between the first and second
carrier frequencies from the first carrier frequency and adding the
spectral distance between the first and second carrier frequencies
to the second carrier frequency.
13. The apparatus of claim 1, wherein the at least one active
channel includes first and second channels, wherein the first and
second channels are respectively associated with first and second
carrier frequencies, and wherein the circuit arrangement is
configured to determine the identified portion of the frequency
band by accessing a lookup table indexed by the first and second
carrier frequencies.
14. The apparatus of claim 1, wherein the RF communications signal
comprises a multi-carrier signal, the apparatus further comprising:
(a) a power amplifier disposed in the signal path and configured to
output a multi-carrier output signal; and (b) a feed forward signal
path coupled in parallel with the signal path; wherein the circuit
arrangement is configured to generate a suppression signal in the
feed forward signal path that, when combined with the multi-carrier
output signal generated by the power amplifier, reduces the IMD
products in the multi-carrier output signal.
15. A power amplifier, comprising: (a) an amplifier circuit
disposed in a main signal path and configured to amplify an RF
input signal disposed in a frequency band to generate an RF output
signal; (b) a scanning receiver coupled to the main signal path and
configured to monitor power on the signal path in a controlled
portion of the frequency band; and (c) a control circuit disposed
in a feed forward path and configured to generate a suppression
signal that, when combined with the RF output signal, suppresses
intermodulation distortion (IMD) products disposed in a selected
portion of the frequency band, the control circuit configured to,
in a first mode, control the scanning receiver to identify at least
one active channel among a plurality of channels in the frequency
band, and, in a second mode, to control the scanning receiver to
monitor IMD products in the selected portion of the frequency band,
wherein the selected portion of the frequency band is associated
with the active channel(s) identified by the scanning receiver.
16. The power amplifier of claim 15, wherein the control circuit is
configured to identify each active channel by detecting an active
signal at a carrier frequency associated with such active
channel.
17. The power amplifier of claim 15, wherein the control circuit is
configured to sequentially tune the scanning receiver to each of a
plurality of carrier frequencies, compare the power signal output
by the scanning receiver at each carrier frequency to a threshold,
and identify an active channel among the plurality of channels
based upon the comparison of the power signal output by the
scanning receiver when tuned to a carrier frequency associated with
such active channel to the threshold.
18. The power amplifier of claim 15, wherein the RF input signal
comprises a multi-carrier communications signal.
19. The power amplifier of claim 15, wherein the at least one
active channel includes first and second channels respectively
associated with first and second carrier frequencies, and wherein
the control circuit is configured to determine the selected portion
of the frequency band by performing a calculation selected from the
group consisting of subtracting a spectral distance between the
first and second carrier frequencies from the first carrier
frequency and adding the spectral distance between the first and
second carrier frequencies to the second carrier frequency.
20. The power amplifier of claim 15, wherein the at least one
active channel includes first and second channels, wherein the
first and second channels are respectively associated with first
and second carrier frequencies, and wherein the control circuit is
configured to determine the selected portion of the frequency band
by accessing a lookup table indexed by the first and second carrier
frequencies.
21. A power amplifier, comprising: (a) an amplifier circuit
disposed in a main signal path and configured to amplify an RF
input signal disposed in a frequency band to generate an RF output
signal; (b) a scanning receiver coupled to the main signal path and
configured to detect an output level of the RF output signal in a
selected portion of the frequency band; and (c) a control circuit
coupled to receive the detected output level from the scanning
receiver, the control circuit further configured to control the
scanning receiver to select as the selected portion of the
frequency band each of a plurality of channels in the frequency
band so as to generate a power signal representative of an output
level of the RF output signal in each of the plurality of
channels.
22. A circuit arrangement for use in a feed forward, multi-carrier
power amplifier system to suppress intermodulation distortion (IMD)
products from an RF communications signal, the circuit arrangement
comprising: (a) a mixer for downconverting an RF carrier signal to
an intermediate frequency (IF) signal, wherein the RF carrier
signal is disposed at a channel among a plurality of channels in a
frequency band, each channel associated with a carrier frequency;
(b) a filter responsive to the IF signal and configured to pass
only a predetermined portion of the IF signal; (c) a detector
responsive to the predetermined portion of the IF signal passed by
the filter, the detector configured to generate a power signal
representative of the power of the portion of the IF signal passed
by the filter; and (d) a processing unit configured to generate a
suppression signal that suppresses IMD products from the RF
communications signal responsive to the power signal.
23. The circuit arrangement of claim 22, wherein the filter
comprises a SAW bandpass filter.
24. The circuit arrangement of claim 22, wherein the detector
comprises a log detector.
25. The circuit arrangement of claim 22, wherein the processing
unit comprises a lookup table identifying at least one IMD location
associated with at least first and second carrier frequencies
located in at least first and second channels.
26. The circuit arrangement of claim 22, wherein the RF
communications signal is modulated according to one of a WCDMA and
CDMA modulation format.
27. A method of suppressing intermodulation distortion (IMD)
products in an RF communication system operating within a frequency
band, the method comprising: (a) analyzing an RF communications
signal communicated by a signal path in the RF communication system
to identify at least one active channel among a plurality of
channels in the frequency band; (b) identifying at least one
portion of the frequency band likely to include IMD products based
upon the identified active channel(s); and (c) suppressing from the
RF communications signal the IMD products at the identified portion
of the frequency band.
28. The method of claim 27, wherein the plurality of channels in
the frequency band are each associated with a non-varying carrier
frequency.
29. The method of claim 27, wherein the frequency band is the
Universal Mobile Telecommunications System (UMTS) frequency
band.
30. The method of claim 27, wherein identifying the at least one
active channel includes detecting an active signal at a carrier
frequency associated with an active channel.
31. The method of claim 30, wherein identifying the at least one
active channel includes: (a) tuning a scanning receiver coupled to
the signal path to a selected carrier frequency; and (b) receiving
a power signal output by the scanning receiver and representative
of a signal power in the signal path at the selected carrier
frequency.
32. The method of claim 31, wherein tuning the scanning receiver
includes sequentially tuning the scanning receiver to each of a
plurality of carrier frequencies, the method further comprising:
(a) comparing the power signal output by the scanning receiver at
each carrier frequency to a threshold; and (b) identifying an
active channel among the plurality of channels based upon the
comparison of the power signal output by the scanning receiver when
tuned to a carrier frequency associated with such active channel to
the threshold.
33. The method of claim 31, further comprising determining whether
the identified active channel(s) constitute a valid channel
configuration.
34. The method of claim 31, further comprising storing a value
associated with the power signal at each carrier frequency.
35. The method of claim 31, wherein suppressing the IMD products
includes: (a) tuning the scanning receiver to the identified
portion of the frequency band; (b) monitoring the power signal when
the scanning receiver is tuned to the identified portion of the
frequency band; and (c) adjusting at least one of a phase and
magnitude of a suppression signal applied to the signal path so as
to reduce the signal power in the signal path within the identified
portion of the frequency band.
36. The method of claim 35, further comprising: (a) identifying a
plurality of portions of the frequency band likely to include IMD
products; and (b) suppressing the IMD products by tuning the
scanning receiver to each identified portion of the frequency band,
monitoring the power signal when the scanning receiver is tuned to
such identified portion of the frequency band, and adjusting at
least one of a phase and magnitude of a suppression signal applied
to the signal path so as to reduce the signal power in the signal
path within the plurality of identified portions of the frequency
band.
37. The method of claim 27, wherein the RF communications signal
comprises a multi-carrier communications signal.
38. The method of claim 27, wherein the at least one active channel
includes first and second channels respectively associated with
first and second carrier frequencies, and wherein identifying the
identified portion of the frequency band includes performing a
calculation selected from the group consisting of subtracting a
spectral distance between the first and second carrier frequencies
from the first carrier frequency, and adding the spectral distance
between the first and second carrier frequencies to the second
carrier frequency.
39. The method of claim 27, wherein the at least one active channel
includes first and second channels respectively associated with
first and second carrier frequencies, and wherein identifying the
identified portion of the frequency band includes accessing a
lookup table indexed by the first and second carrier
frequencies.
40. The method of claim 27, wherein the RF communication system
includes a multi-carrier power amplifier disposed in the signal
path and configured to output a multi-carrier output signal, and a
feed forward signal path coupled in parallel with the signal path,
and wherein suppressing the IMD products includes generating a
suppression signal in the feed forward signal path that, when
combined with the multi-carrier output signal generated by the
power amplifier, reduces the IMD products in the multi-carrier
output signal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 60/273,659, filed Mar. 6, 2001 by
Thomas A. Bachman, II et al., which application is incorporated by
reference herein.
FIELD OF THE INVENTION
[0002] This invention relates generally to radio frequency (RF)
power amplifiers, and more particularly to power amplifiers
incorporating a linearization scheme, e.g., feed forward
multi-carrier power amplifiers (MCPA's) that attempt to reduce or
eliminate intermodulation distortion (IMD) products.
BACKGROUND OF THE INVENTION
[0003] Ideally, RF power amplifiers would act linearly, faithfully
reproducing an amplified RF signal at their output with no
distortion. Requirements for efficiency, however, can lead to
operating amplifiers close to saturation, where non-linearities
create unwanted IMD. IMD products may cause interference,
disrupting the proper transmission and reception of RF signals,
particularly in adjacent channels. Numerous techniques have been
developed to reduce IMD products from amplified RF signals,
including feed forward, predistortion, and linear amplification
with non-linear components (LINC).
[0004] Recent surges in demand for wireless solutions have led to
new frequency bands to increase capacity, such as, for example, the
Universal Mobile Telecommunications System (UMTS) developed by the
European Telecommunications Standard Institute for delivering 3G
(third generation) services. Modern transmission protocols, such as
UMTS, demand high linearity to prevent radio frequency energy in
one band from spilling over and interfering with other proximate
channels, but often have high Peak-to-Average Power Ratio (PAR)
carrier signals that make efficient linear amplifiers difficult to
design. This energy leakage can undesirably degrade the
signal-to-noise (SNR) ratio or bit-error rate (BER) of the
proximate frequency channels.
[0005] In practice, it is very difficult and often unnecessary to
eliminate completely all IMD products for a selected center
frequency. A certain tolerable level of IMD products is acceptable.
When the terms "eliminate" or "reduce" are used herein with
reference to the IMD products, it is understood that the IMD
products should be suppressed below a certain tolerable level, even
though they may not be entirely eliminated.
[0006] One common technique to reduce IMD to acceptable levels is
feed forward correction, whereby the IMD products are isolated and
manipulated so that at the final summing point the IMD products
substantially cancel out. However, the input signal pattern can be
unpredictable, often making the IMD products difficult to locate.
One way to address this problem is to inject an artificial signal,
conventionally called a pilot tone or pilot signal, to simulate the
unwanted distortion to be removed. At the output, a pilot signal
receiver detects the simulated distortion, and the amplifier is
aligned in accordance with a signal representative of the pilot
signal receiver output. Significantly, these pilot signal receivers
do not detect and measure the actual non-linear distortion
components. Instead, they detect and measure the simulated
distortion based on an injected pilot signal so that, at the final
summing point, the simulated distortion is canceled out with the
intent that IMD will also cancel out, leaving only the amplified
carrier signals.
[0007] Some pilot tone systems inject the pilot signal into the
main signal before the carriers are amplified; others inject the
pilot signal after the carriers have been amplified. In either
case, the distortion products contain "artificial" distortion
products in addition to the non-linear distortion products created
by the power amplifier. As a result pilot tone systems suffer from
several drawbacks. First, pilot tone systems do not actually detect
and eliminate the actual distortion produced by the power
amplifier. Because they detect distortion created by an
artificially injected signal and not the actual distortion created
by the power amplifier, the actual distortion may not be entirely
cancelled and the artificial distortion may leak into the output.
Moreover, circuit complexity, size, and cost are increased because
the pilot tone circuit must include a pilot signal generator, and a
pilot signal injector, among other things. In addition, the pilot
signal receiver may need to be tightly synchronized with the
transmitter to obtain optimum cancellation of the distortion
products generated by the pilot signal and the power amplifier.
[0008] In another technique, the locations of the distortion
products can be calculated without the use of a pilot tone or
signal. In one such technique, an amplified WCDMA carrier
containing both in-band frequency components and undesired spectral
regrowth components is downconverted to baseband, digitized by an
analog-to-digital converter, and then spectrally analyzed in a
digital signal processor (DSP) to locate the carrier frequency and
to determine the locations of the undesired distortion components.
However, this approach is often undesirable because the DSP and
related circuitry increase the overall cost and complexity of the
power amplifier.
[0009] Therefore, a need exists for a power amplifier that
incorporates an IMD detector circuit that is relatively simple in
design and that can be manufactured at a relatively low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The invention and further objectives and advantages thereof
may best be understood by reference to the following description
taken in conjunction with the accompanying drawings, in which:
[0011] FIG. 1 is a functional diagram of a feed forward
multi-carrier power amplifier circuit in accordance with one aspect
of the present invention;
[0012] FIG. 2 is a functional diagram of a scanning receiver in
accordance with one aspect of the present invention;
[0013] FIG. 3 is a diagrammatic flow chart of a channel scan
algorithm in accordance with one aspect of the present invention;
and
[0014] FIG. 4 is a diagrammatic flow chart of another channel scan
algorithm in accordance with another aspect of the present
invention.
DETAILED DESCRIPTION
[0015] Although the invention will be described next in connection
with certain embodiments, it will be understood that the invention
is not limited to those particular embodiments. On the contrary,
the description of the invention is intended to cover all
alternatives, modifications, and equivalent arrangements as may be
included within the spirit and scope of the invention as defined by
the appended claims.
[0016] The present invention relates to a method and apparatus for
locating and suppressing intermodulation distortion (IMD) products
in a power amplifier system. In the embodiments described
hereinafter, IMD products are suppressed in a feed forward
multi-carrier power amplifier system. However, as will become
apparent below, the principles of the invention may apply to
single-carrier power amplifier systems, as well as to power
amplifier systems incorporating other linearization schemes, e.g.,
predistortion amplifiers, Envelope Elimination and Restoration
(EER) amplifiers, and various LINC amplifier designs, etc.
Implementation of the herein-described techniques for suppressing
IMD products within the context of other linearization schemes
would be well within the abilities of one of ordinary skill in the
art having the benefit of the instant disclosure.
[0017] As used herein, IMD products refer to the distortion
products created by carrier signals. In the illustrated embodiment,
a main signal path and a feed forward path receive a multi-carrier
communications signal typically including multiple RF signals,
which are located in a frequency band having a stationary constant
channel configuration. In a specific aspect of the present
invention, the communication protocol is UMTS. The RF signals are
amplified by a main amplifier on the main signal path to produce
amplified RF signals and creating undesired IMD products. The
amplified RF signals and undesired IMD products are coupled to a
scanning receiver. The scanning receiver typically includes a
frequency synthesizer, a mixer, a filter, and a detector. A
processor connected to the scanning receiver tunes the frequency
synthesizer to a desired location, and the log detector outputs a
signal representative of the power in a portion of the frequency
band based on a local oscillator signal from the frequency
synthesizer.
[0018] The power signal output by the scanning receiver may be used
to either scan for active channels, or to drive an error correction
loop in the feed forward amplifier system to optimally suppress IMD
products. An active channel is a channel that contains an RF signal
at its carrier frequency.
[0019] When the scanning receiver is used to scan for active
channels, at least one portion of a frequency band is typically
identified as being likely to include IMD products based upon the
active channels that are identified during scanning. IMD product
suppression can then be directed to the identified portion of the
frequency band.
[0020] FIG. 1 shows a functional diagram of a typical feed forward
multi-carrier power amplifier (MCPA) circuit with a correction
circuit that includes a control circuit and a scanning receiver
144. It should be understood that feed forward circuits are well
known in the art, and that the feed forward circuit shown in FIG. 1
is merely exemplary and that numerous variations of the feed
forward circuit provided in FIG. 1 could be employed without
departing from the spirit and scope of the present invention.
According to one aspect of the present invention, the typical
circuit generally includes an input 100, a main signal path 102, a
feed forward path 104, and an output 112. The circuit further
includes a carrier correction loop (CCL) 106, an error correction
loop (ECL) 108, and a scanning receiver path 110. On the feed
forward path 104, there is provided a feed forward delay filter
118, a feed forward attenuator 120, a feed forward phase shifter
122, and a feed forward amplifier 124. On the main signal path 102,
there is provided a main attenuator 134, a main phase shifter 136,
a main amplifier 138, and a main delay filter 140. Note that the
feed forward attentuator 120 and feed forward phase shifter 122 may
be incorporated into the feed forward amplifier 124, and the gain
and phase of the feed forward amplifier 124 may be controlled by
gain and phase control lines (not shown). Similarly, the main
attentuator 134 and main phase shifter 136 may be incorporated into
the main amplifier 138.
[0021] The input 100 receives radio frequency (RF) carrier signals
(also collectively referred to herein as a multi-carrier
communications signal), and an input carrier coupler 114 couples
the RF carrier signals onto both the main signal path 102 and the
feed forward path 104. Alternatively, a splitter (not shown) may be
used to provide the RF carrier signals onto the main signal path
102 and the feed forward path 104. The RF carrier signals lie in
any frequency band which has a constant channel configuration,
i.e., where each channel has a non-varying carrier frequency and a
slow varying average power. An example of a frequency band having a
constant channel configuration is UMTS, though other frequency
bands may have constant channel frequency configurations. The
present invention contemplates any frequency band that has a
constant channel configuration as described herein.
[0022] The UMTS frequency band is 60 MHz wide, spanning the
2110-2170 MHz frequency range, where each channel has a bandwidth
of 5 MHz. Up to four channels may be active simultaneously subject
to restrictions on center frequencies detailed in the UMTS
specification. A channel is active when RF signals are present at
its carrier frequency. In the UMTS band, the carrier frequency is
located in the center of the channel. Furthermore, users are added
in a Code Division Multiple Access (CDMA) configuration, setting up
an input signal to the power amp that is stationary in frequency
and maintained between power levels. This constant (non-varying)
input arrangement facilitates the use of the input signal to drive
the Error Correction Loop (ECL), and often replacing the need for
an injected pilot tone configuration. Other modulation formats are
expressly contemplated by the present invention, so long as they
are operable in a frequency band having a constant channel
configuration such as UMTS.
[0023] Referring again to FIG. 1, the RF carrier signals on the
main signal path 102 may be attenuated by the main attenuator 134
and phase shifted by the main phase shifter 136, but not
necessarily in that order. Optionally, a CCL power detector 150 may
be provided on the feed forward path 104 to monitor the power level
of the signals after the carriers have been subtracted from the CCL
106. Control of the main attentuator 134 and the phase shifter 136
may be under microprocessor control or any other suitable interface
capable of monitoring the input power detector 116 and adjusting
the main attentuator 134 and the phase shifter 136 in accordance
with the output of the CCL power detector 150. The voltage from the
CCL power detector 150 may be used to adjust the main attenuator
134 and the phase shifter 136 on the main signal path 102 to obtain
maximum carrier cancellation out of the CCL 106.
[0024] Optionally, an input power detector 116 may be provided on
the main signal path 102 to monitor the input power levels. For
example, if the power level of a carrier signal exceeds a desired
threshold, the input power detector 116 may be used to trigger an
error condition, such as a reset or power down.
[0025] After the RF carrier signals have been attenuated and phase
shifted, they are amplified by the main amplifier 138 to generate a
multi-carrier output signal. For efficiency, the main amplifier 138
should be driven as close to saturation as possible, while
maintaining necessary headroom for the high PAR. As a result, the
main amplifier 138 produces amplified RF carrier signals and
undesired IMD products. If the RF carriers, for example, lie in
adjacent frequency channels, the IMD products from one frequency
channel may spill over into other frequency channels. This effect
becomes more pronounced the closer the main amplifier 138 is driven
to saturation.
[0026] Next, the amplified RF carrier signals and undesired IMD
products are time delayed by the main delay filter 140 to produce
delayed amplified RF carrier signals and delayed amplified IMD
products on the main signal path. Note that other suitable delay
elements may be used to time-delay signals. The time delay is
selected such that the amplified RF carrier signals and associated
IMD products appear in the main signal path 102 at substantially
the same time the adjusted carrier signals and associated IMD
products from the feed forward amplifier 124 are coupled onto the
main signal path 102.
[0027] Meanwhile, on the feed forward path 104, a feed forward
delay filter 118 delays the RF carrier signals such that the RF
carrier signals appear in the feed forward path 104 at
substantially the same time the attenuated sample of the amplified
RF carrier signals (and associated IMD products) are coupled onto
the feed forward path 104 by a feed forward CCL coupler 130.
[0028] The carrier correction loop (CCL) 106 couples the amplified
RF carrier signals and associated IMD products on the main signal
path 102 onto the feed forward path 104 at the output of the feed
forward delay filter 118. The CCL 106 includes (1) a main CCL
coupler 126 which couples the amplified RF carrier signals and
associated IMD products on the main signal path 102 onto the CCL
106, (2) a CCL attentuator 128 for attenuating the amplitude of the
coupled signals, and (3) a feed forward CCL coupler 130 which
couples the amplified RF carrier signals and undesired IMD products
onto the feed forward path 104 at the output of the feed forward
delay filter 118. The phase of the amplified RF carrier signals
should be inverted with respect to the phase of the delayed (input
sample) RF carrier signals on the feed forward path after the feed
forward delay filter 118.
[0029] The CCL attenuator 128 attenuates the coupled signals such
that the amplitude of the amplified RF carrier signals is
substantially equal to the amplitude of the delayed (input sample)
RF carrier signals on the feed forward path, in order to obtain
maximum carrier cancellation. Attenuation resulting from the main
CCL coupler 126 and the feed forward CCL coupler 130, as well as
the gain of the main amplifier 138, should be taken into
consideration when selecting the attenuation factor for the CCL
attenuator 128. After coupling by the feed forward CCL coupler 130,
the two out-of-phase carrier signals cancel each other such that
primarily isolated IMD products remain on the feed forward path
104, though some level of carrier products may also be present.
These isolated IMD products are adjusted in magnitude and phase
with respect to the amplified IMD products at the output of the
main amplifier 138, so that the two signals will cancel each other
when combined.
[0030] The isolated IMD products are presented to the feed forward
attenuator 120, the feed forward phase shifter 122, and the feed
forward amplifier 124. Note that the feed forward attenuator 120
and feed forward phase shifter 122 may be incorporated into the
feed forward amplifier 124. The amplitude of the isolated IMD
products may be attenuated by the feed forward attenuator 120, and
the phase of the isolated IMD products may be shifted by the feed
forward phase shifter 122, but not necessarily in that order. The
feed forward attenuator 120 and feed forward phase shifter 122 are
under the control of a processing unit 240 as shown in FIG. 2, as
will be explained in more detail later. The processing unit 240
could be comprised of more than one unit. For example, the
processing unit 240 could include a scanning mode unit and a
correction mode unit, where the scanning mode unit locates carrier
activity across the frequency band, and where the correction mode
unit drives the error correction loop 106 to suppress the undesired
IMD products on the main signal path 102. These units may be
comprised of any combination of analog and/or digital devices, such
as an analog processor and/or a microprocessor.
[0031] The attenuated and phase-shifted IMD products are amplified
by a feed forward amplifier 124 to generate a suppression signal.
The gain of the feed forward amplifier 124 is selected such that
the IMD products are substantially eliminated at the output 112.
The feed forward amplifier 124 is typically driven well below
saturation to avoid creating non-linear distortion products in the
ECL 108.
[0032] The feed forward amplifier 124 produces amplified IMD
products whose phase is inverted with respect to the phase of the
delayed amplified IMD products on the main signal path 102. The
amplitudes of the amplified IMD products and the delayed amplified
IMD products are substantially identical. Because they are also
phase inverted, when they are coupled by the main ECL coupler 132
onto the main signal path 102, the amplified IMD products and the
delayed amplified IMD products substantially cancel each other so
that IMD products are essentially eliminated from the main signal
supplied to the main output 112.
[0033] The resultant amplified RF carrier signals (and their
associated IMD products, if any) are coupled onto a scanning
receiver loop 110 by a scanning receiver coupler 142. Optionally, a
splitter 146 may provide the amplified RF carrier signals to both
the scanning receiver 144 and to an output power detector 148. The
scanning receiver 144 produces an output voltage at the scanning
receiver input control 232 connected to the processing unit shown
in FIG. 2. Optionally, the output power detector 148 detects the
power of the amplified RF signals. For example, the output power
detector 148 may monitor the output power of the main amplifier 138
for abnormalities, and trigger a fault isolation loop when, for
example, too much power is detected.
[0034] Referring now to FIG. 2, there is shown a functional diagram
of the scanning receiver 144. The scanning receiver 144 generally
includes a scanning receiver input 200, a mixer 204, a frequency
synthesizer 202, and a scanning receiver input control 232. In one
embodiment, the frequency synthesizer 202 is a direct digital
synthesizer, though other frequency synthesizers could be employed.
In one embodiment, the frequency synthesizer 202 includes a
phase-locked loop (PLL) synthesizer 206, a reference oscillator
208, a loop filter 210, and a voltage controlled oscillator (VCO)
212. The PLL synthesizer 206 is driven by the highly stable
reference oscillator 208, which optionally may be a temperature
controlled crystal oscillator (TCXO), so that the VCO 212 will hold
its assigned frequency over wide temperature gradients. In one
aspect of the present invention, the reference oscillator 208
oscillates at 10 MHz, but other frequencies may be employed
depending on the PLL synthesizer 206 design specifications, and
other considerations. The loop filter 210 is connected between the
PLL synthesizer 206 and VCO 212 and cleans the PLL synthesizer 206
output to prevent rippling or modulation of the VCO 212. The loop
filter 210 may be selected in accordance with design specifications
such as bandwidth, phase margin, lock time, settling time, and loop
order. For example, in one embodiment, the loop filter 210 is a
third order filter with a bandwidth of 10-20 kHz.
[0035] The output of the VCO 212 may be optionally provided to an
LO amplifier 214, which amplifies the output of the VCO 212 to a
range necessary to drive the LO input of the mixer 204. For
example, in one embodiment, the mixer 204 may be a Hittite
HMC175MSB mixer, which has a typical LO drive level of 13 dBm.
However, it is understood that any other suitable mixer or
frequency converter could be used. The mixer 204 downconverts a
portion of the frequency band based on a known frequency from the
VCO 212 to an intermediate frequency (IF). In one embodiment where
the carriers lie in a UMTS band, the IF frequency is 246 MHz. It is
expressly understood that any other suitable IF frequency could be
selected. For example, to detect the power of a portion of the
frequency band around 2115 MHz, the VCO would be tuned to provide
an LO of 2115 MHz.+-.246 MHz.
[0036] An IF filter 220 passes only a selected portion of the IF
signals and produces passed IF signals. In one embodiment, the IF
filter 220 is a SAW bandpass filter having a pass bandwidth of 300
kHz centered around 246 MHz. It should be understood that the IF
filter 220 can be any type of filter, and those of ordinary skill
will appreciate that the type and number of filters will be driven
by considerations such as over what range RF signal activity is to
be measured, over what range IMDs are to be cancelled, and other
considerations.
[0037] An IF amplifier 222 compensates for insertion loss and sets
the drive level into a log detector 228. In one embodiment, two
filtering and gain stages are provided to achieve high
selectability (i.e., ability to resolve low power signals in the
presence of high level carriers) and to compensate for insertion
loss. Thus, in one embodiment, the IF filter 220 includes two SAW
(surface acoustic wave) band pas filters having a pass bandwidth of
300 kHz centered around 246 MHz and further includes two amplifiers
to compensate for insertion loss through the SAW band pass filters.
It is understood that other filtering and gain stage combinations
may be employed without departing from the spirit and scope of the
present invention.
[0038] Next, the filtered IF signals are converted to a voltage
representative of a characteristic (e.g., the voltage, dBV or
power, dBm or dBW) of the IF signal by the log detector 228. Note
that the input characteristic of a detector is sometimes expressed
as voltage (dBv), but may also be expressed in terms of power (dBm
or dBW). In one embodiment, the log detector 228 is a high dynamic
range demodulating logarithmic amplifier such as an Analog Devices
AD8310, though any other suitable detector may be employed. In one
embodiment, peak detection using a capacitor (not shown) may be
used to remove the a/c components from the output of the log
detector 228.
[0039] Optionally, the output voltage from the log detector 228 may
be provided to an operational amplifier (op amp) 230 for scaling
and signal conditioning, if desired. According to one aspect of the
present invention, the op amp 230 may be configured as a buffer,
gain stage or a 1-pole or 2-pole active low pass filter for best
closed-loop performance. It should be understood, though, that the
use of the op amp 230 is optional, and certain design requirements
may not require filtering or scaling of the output voltage from the
log detector 228.
[0040] Finally, the scanning receiver input control (or power
signal) 232 is a voltage (DC volts) representative of the power
(expressed in dBV or dBm) of a portion (determined by the IF filter
220) of the frequency band (UMTS in one embodiment). The scanning
receiver input control 232 is provided as an input to the
processing unit 240 as shown in FIG. 2. Alternative representations
of power may be utilized for control 232, e.g., current, a digital
value, etc.
[0041] The processing unit 240 includes all necessary (at least
four) control lines: an attenuator output control 234, a phase
shifter output control 236, a PLL output control 238, and a
scanning receiver input control 232. The scanning receiver input
control 232 to the processing unit 240 is connected to the scanning
receiver 144 shown in FIG. 1. The PLL output control 238 to the
processing unit 240 is connected to the PLL synthesizer 206. The
attenuator output control 234 is connected to the feed forward
attenuator 120, and the phase shifter output control 236 is
connected to the feed forward phase shifter 122 shown in FIG.
2.
[0042] Still referring to FIG. 2, the processing unit 240 has two
modes of operation: a scanning mode and a correction mode. In the
scanning mode, which may occur at power up (i.e. when power is
supplied to the circuit) or at regular intervals during operation,
the processing unit 240 determines which channels of the frequency
band are active. In the correction mode, the processing unit 240
calculates a first location of IMD products based on the carrier
frequencies in the channels found to be active in the scanning
mode, then tunes the PLL synthesizer 206 to the first location plus
the desired IF, and, responsive to the voltage developed on line
232 by the scanning receiver 144, adjusts the feed forward
attenuator 120 and feed forward phase shifter 122 until the IMD
products in the main signal path are optimally suppressed.
[0043] FIG. 3 is a diagrammatic flow chart of the scanning mode in
accordance with one embodiment of the present invention. In the
scanning mode, the processing unit 240 "hops" across the channels
to determine which are active. In other words, the processing unit
240 need not incrementally sweep or step across the entire channel.
Rather, because the channel configuration is constant, the
processing unit 240 may be instructed to hop from one carrier
frequency to another, skipping frequencies in between. The channel
hopping procedure described herein is discussed next.
[0044] First, at power up, or at some regular time interval during
normal operation (300), the processing unit 240 tunes the PLL
synthesizer 206 to drive the VCO 212 to the center frequency of the
first channel (CH.dbd.CH1) (302) plus a predetermined IF frequency
(304). In the UMTS frequency band, the bandwidth for transmission
is 2110 MHz=2170 MHz. In one embodiment, the IF frequency is 246
MHz, though any other suitable IF frequency may be desired. After
tuning the VCO 212, the scanning receiver 144 produces an output
voltage which is provided to the processing unit 240 by scanning
receiver input control 232 (306).
[0045] In one aspect of the present invention, the processing unit
240 compares the output voltage from scanning receiver input
control 232 to a threshold (308). If the voltage exceeds the
threshold, then the channel represented by CH is deemed to be
active (308). Otherwise, the channel represented by CH is deemed to
be not active (308). In one embodiment, the output voltage is
digitized by, for example, an analog-to-digital converter (not
shown), and the value representative of the digitized voltage is
compared against a threshold digital value. The result of the
comparison from the channel under consideration is stored in a
memory device such as a RAM, or any other suitable device. The
active-channel threshold may be fixed or variable. Next, the
processing unit 240 checks whether the channel being analyzed is
the last channel in the frequency band (310). If the last channel
is not present, the processing unit 240 calculates the next center
frequency (312), and (304 through (310) are repeated until the last
channel is scanned.
[0046] The UMTS Specification has restrictions on multi-carrier
frequency locations, such that when one is found, rather than
sweeping the scanning receiver 144 across the entire channel in
incremental steps, the processing unit 240 may hop ahead to the
next center frequency. Because the channel configuration is
constant, the locations of the carrier frequencies are known a
priori, obviating the need to scan across the entire channel.
[0047] When the last channel is scanned, the processing unit 240
optionally may determine whether the channel map is a valid
configuration (314). If the channel map is not valid (316), the
processing unit 240 may rescan for active carriers (302). By way of
example only, the channel map in accordance with the UMTS band is
not valid if carrier activity is detected in more than four
channels. Whatever the frequency band, the processing unit 240
determines whether the detected carrier patterns conform with the
requirements of that particular frequency band. In one embodiment,
the processing unit 240 compares the comparison results (active or
not active) of each visited channel stored in memory against a
predetermined channel map configuration stored in memory, for
example. Alternatively, the processing unit 240 executes a sequence
of instructions to determine whether the channel map is valid.
[0048] Once the scanning mode is complete, and, if performed, the
channel map is found to be valid, the processing unit 240 next
enters a correction mode to locate the IMD products and to suppress
them (318).
[0049] These calculations may be performed by the processing unit
240 or by a separate unit (not shown) connected to the processing
unit 240, as explained earlier. One advantage of the present
invention is that it provides a versatile voltage from the scanning
receiver 144 representative of the power in a selected portion of
the frequency band. It is expressly understood that there are
numerous ways to use the output voltage to locate an active channel
or to drive a correction loop. Those of ordinary skill will
appreciate the flexibility the present invention offers by
providing a voltage representative of the power in a portion of a
frequency band, whether that portion comprises a carrier signal,
IMD products, or no signal at all.
[0050] The IMD locations may be determined either by performing a
calculation according to any number of algorithms, or they may be
stored in a lookup table in a memory device, such as an EPROM or
other suitable device. For example, the lookup table may contain
combinations of active channels and IMD locations associated with
each combination of active channels. The calculations may be based
on the carrier frequencies of the active channels. For example, in
one embodiment, to calculate IMD locations associated with two
active channels, the spectral difference between the center
frequencies of two active channels is taken, and then subtracted
from the center frequency of the first active channel to obtain a
first IMD location. Optionally, the spectral difference could also
be added to the center frequency of the second active channel to
obtain a second IMD location. For example, consider one UMTS
channel is active with a center frequency at 2132.5 MHz, and a
second channel is active with a center frequency of 2147.5 MHz.
Subtracting the difference between these two center frequencies
from 2132.5 yields a first IMD location of 2117.5 MHz. Adding this
difference to 2147.5 yields a second IMD location of 2162.5 MHz.
The IMD locations may be stored in a memory device, such as in the
register memory of the processing unit 240, an EPROM device, or any
other suitable device.
[0051] In an alternative embodiment, a lookup table indexed by
carrier frequencies may be formed containing combinations of active
channels and associated IMD locations. For the example above, at
least two IMD locations are 2117.5 MHz and 2162.5 MHz. For active
channels at 2112.5 and 2117.5 MHz, the IMD locations are 2107.5 MHz
and 2122.5 MHz. Other or different locations may be optimally
determined by analyzing where the IMD products appear when certain
combinations of channels become active.
[0052] In some embodiments, e.g., in single-carrier environments, a
scan of active channels by scanning receiver 144 may identify as
few as one active channel. In such instances, the identification of
an IMD location may be based upon the location of a single active
channel.
[0053] The PLL synthesizer 206 tunes the VCO 212 to the first IMD
location of 2117.5 MHz. A portion of the band centered about this
frequency is selected by the IF filter 220, and the log detector
228 outputs a voltage representative of the power at this selected
portion of the band. As the feed forward attenuator 120 and feed
forward phase shifter 122 are optimized, the voltage representative
of the power at a portion of the frequency centered about 2117.5
MHz should decrease until it falls below a predetermined threshold.
During optimization, the power around the second IMD location may
be measured. The attenuator 120 and phase shifter 122 may be
adjusted, until the power at either location or both locations is
reduced below a predetermined threshold.
[0054] In this fashion, the scanning receiver 144 may correct for
the first IMD location only or for the second IMD location only or
for both. In yet another embodiment, such as when two adjacent
channels and third nonadjacent channel are active, a third IMD
location which lies between the two adjacent channels and third
non-adjacent channel may be calculated, and the scanning receiver
144 may hop from one IMD location to another to find an optimal
setting for the feed forward attenuator 120 and feed forward phase
shifter 122. For example, after optimizing one IMD location, it may
be found that another IMD location has not been optimally
suppressed. It may be necessary to "back off" the first IMD
location so that both the IMD products at both first and second IMD
locations are optimally suppressed. It is to be expressly
understood that there are numerous ways of locating and eliminating
the IMD locations without departing from the spirit and scope of
the present invention. The goal is to suppress the undesired IMD
products in the main signal path below an acceptable threshold. For
certain combinations of active channels, it may be adequate to find
one IMD location and optimize based only on the power detected at
the first IMD location; in others it may be necessary to monitor
the power at more than one IMD location by hopping from one to the
other until the IMD products at both locations are optimally
suppressed.
[0055] Returning now to FIG. 3, after the IMD locations are
calculated (318), the processing unit 240 tunes the frequency
synthesizer 202 to a frequency representative of an IMD location
(320). The IMD products are optimally suppressed (322) by adjusting
the feed forward attenuator 120 via attenuator output control 234,
and by adjusting the feed forward phase shifter 122 via phase
shifter output control 236 until the output voltage from the
scanning receiver 144 falls below a desired threshold. In one
embodiment, a gradient search or dither type (sample and step)
algorithm is used to drive the feed forward attenuator 120 and feed
forward phase shifter 122, though any other suitable algorithm may
be used.
[0056] FIG. 4 illustrates a diagrammatic flow chart of yet another
scanning mode in accordance with another aspect of the present
invention. Like before, at power up or at various time intervals
during normal operation (300), the center frequency of the first
channel is determined (302), and the frequency synthesizer 202 is
tuned to the first channel's center frequency CH+IF (304). Next,
the scanning receiver output voltage 232 is read (306), which
represents the power detected in a portion of the band around the
center frequency. A value representative of the voltage 232 is
stored in a memory device (400), such as in RAM or any other
suitable memory device. Next the scanning mode checks for whether
the last channel in the frequency band has been scanned (402). If
the last channel has not been scanned, the center frequency for the
next channel is calculated (404), and the frequency synthesizer 202
is tuned to the next channel's center frequency+IF (304), and so
forth.
[0057] In one aspect of the present invention, when all of the
channels have been scanned, the stored results are compared against
predetermined threshold values (406) by the processing unit 240.
The predetermined threshold values may be identical for all
channels or may represent different values for different channels.
In other words, the power required for on e channel to be active
may be different from the power required for another channel to be
active. In one embodiment, these threshold values are stored in a
lookup table.
[0058] The processing unit 240 reads the stored value representing
whether a particular channel is active, and compares that value to
the corresponding threshold value in the lookup table. If the
stored value exceeds the threshold value, the representative
channel is considered active.
[0059] Next, in one embodiment, the channel map configuration is
checked for validity (314). In another embodiment, the validity
check is not performed at all. Typically, the validity check is
performed before the memory comparing step (406). If the channel
map is not a valid configuration, the channel scan is repeated
(302). Otherwise, the correction begins, and the processing unit
240 calculates the location of IMD products associated with an
active channel (318), tunes the frequency synthesizer 202 to a
frequency representative of a first location of IMD products (320),
and drives the error correction loop elements (322) until the IMD
products are optimally suppressed.
[0060] Still with reference to FIG. 4, in another aspect of the
present invention, the comparison step (406) may be performed after
the storing step (400). In other words, the scanning loop (304,
306, 400, 402, 404) need not complete before the status of each
channel is determined. The voltage (or value) representative of the
power of each channel under consideration may be compared with a
threshold voltage (or value if the voltage is digitized) by a
comparator for example immediately after the voltage is received
from the scanning receiver 144 via the scanning receiver input
control 232. In this situation, no memory devices are typically
needed to store the detected voltages from the scanning receiver
144. Rather, the detected voltages compared to a threshold voltage
immediately after detection by the scanning receiver 144, and the
result of this comparison may be stored in a memory device as a
value representative of the comparison result (such as a logical 0
if the channel is not active, or a logical 1 if the channel is
active). Values representative of a threshold voltage may be
stored, for example, in a lookup table in an EPROM or other
suitable memory device.
[0061] In another aspect of the invention the carriers can be
suppressed at the ECL input 130 (FIG. 1) based on IMD cancellation.
In other words, the CCL attenuator 134 (FIG. 1) and CCL phase
shifter 136 (FIG. 1) can be varied for best IMD cancellation,
instead of best carrier cancellation, assuming stable operation of
the power amplifier can be maintained.
[0062] Various other modifications may be made to the
herein-described embodiments without departing from the spirit and
scope of the invention. For example, a scanning receiver may be
used in connection with linearization techniques other than feed
forward correction (e.g., predistortion) to suppress any IMD
products identified as a result of the identification of one or
more active channels by the scanning receiver. Also, it will be
appreciated that a wide variety of alternate circuit arrangements,
including various alternate electronic components, layouts and the
like, may be used consistent with the invention. Therefore, the
invention lies in the claims hereinafter appended.
* * * * *